Turbine engines are used as the primary power source for various aircraft applications. Most turbine engines generally follow the same basic power generation process. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more ambient air into the engine and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, electrical generators, or other devices.
Since turbine engines provide power for many primary and secondary functions, it is important to optimize both the engine service life and the operating efficiency. Although hotter combustion gases typically produce more efficient engine operation, the high temperatures create an environment that promotes oxidation and corrosion. For this reason, diverse coatings and coating methods have been developed to increase the operating temperature limits and service lives of the high pressure turbine components, including the turbine blade and vane airfoils.
One category of conventional airfoil coatings includes platinum aluminide coatings. These coatings may be applied onto surfaces of turbine blades, vanes, and other components to protect against oxidation and corrosion attack. Platinum aluminide coatings provide protection as a result of selective oxidation of aluminum to form an alumina (Al2O3) scale that grows very slowly at high temperatures by a diffusion process. One method for applying a platinum aluminide coating includes depositing aluminum and platinum onto the component substrate. Modification of the platinum aluminide alloy with other elements such as silicon, hafnium, and yttrium, sometimes referred to as “active elements,” enhances the alloy's corrosion resistance properties. However, many active elements rapidly oxidize, and it is advantageous to incorporate such elements in their elemental form in order to provide adequate oxidation protection. Further, there are some inherent difficulties in providing an adequate distribution of active elements in a platinum aluminide diffusion coating.
There is a need for platinum aluminide coatings that have active elements adequately distributed therein. There is also a need for methods of forming such coatings in an atmosphere in which oxidation of the platinum, aluminum, or any active elements is prevented.